The invention is directed to the use of silver-free palladium alloys as materials for baking on dental ceramics.
The complex requirements which have to be fulfilled from fixed dental prosthesis work are the reason why crowns and bridges to a great extent are made from metal. Metallic dental prosthesis predominantly for esthetic reasons are veneered with dental ceramic so that the alloys must be compatible with the current types of dental ceramic. There have proven good for these uses high gold content alloys having about 60-90% gold, about 1-15% palladium, about 1-15% platinum, additionally silver, tin, indium and/or iron and grain refining elements such as iridium, ruthenium and/or rhenium. The high price of noble metals and the cost explosion in public health in recent years, however, have led to the development of alloys with a reduced gold content. Such alloys made of gold-palladium and gold-palladium-silver, however, always contain about 50-60% gold. Silver-containing alloys thereby frequently cause a green discoloration of the dental ceramic in the firing-on process and are seldom used in practice.
Therefore, in recent years there have been developed silver-free palladium based alloys which, because of the lower density and favorable price of palladium, represent the economical type of noble porcelain fused to metal alloy. A further price reduction is only possible by a reduction of the palladium content. Alloys based on palladium-cobalt (e.g. German OS No. 3324987 and Boyajian U.S. Pat. No. 4,261,744), especially in the range of lower palladium concentrations (about 75% and less), show bubble formation and discoloration in the veneering ceramic. The formation of bubbles may be suppressed by adding zinc, aluminium, or silicon, but these alloys then show solid state transformations which create problems in regard to the accuracy of fit of the dental prosthesis.
Furthermore, there are known numerous palladium-copper alloys with one or more of alloying elements tin, indium, cobalt, and grain refining additives of ruthenium and rhenium (e.g. German OS No. 3316595 and related Wagner U.S. application Ser. No. 607,701 filed May 7, 1984 and now allowed, German OS No. 3244802, German OS No. 3146794, German OS No. 3239338, and German OS No. 3247398). All of these alloys additionally contain gallium as an alloy component since this clearly improves the castability of the alloys. Without gallium, the casting results of these alloys can only be reproduced with difficulty, and frequently a formation of voids is observed. Gallium greatly lowers the solidus temperature and widens the melting range. Therefore the high-temperature-strength during baking of the ceramic is reduced, and there is an increased possibility of getting undesired segregations to be avoided upon solidification. Besides, gallium causes a strong increase in hardness. Gallium-containing alloys exhibit a heterogeneous structure and, therefore, are susceptible to corrosion. In addition an increased tendency to bubble formation in the ceramic veneer is observed particularly in alloys with low palladium concentrations.
The known palladium-copper based alloys usually reveal a coefficient of thermal expansion less than 14.0.times.10.sup.-6 /K so that the compatibility with some new higher expanding dental ceramics no longer is guaranteed.
Therefore, it was the problem of the present invention to develop silver-free palladium alloys as materials for baking on dental ceramics which should be as cheap as possible, extra hard, and compatible with the known dental ceramics. Besides, they should be readily workable even without the addition of gallium and should reveal a solidus temperature exceeding 1150.degree. C.
Unless otherwise indicated, all parts and percentages are by weight.